The present invention relates to a dynamic mold seal for use in an injection molding machine, which seal prevents leakage of molten material at the parting line between the molds. The dynamic mold seal of the present invention has particular utility in molding systems for manufacturing compact discs (CDS), digital video discs (DVDs), thinwall containers, and other thinwall articles.
Sometimes during injection, a separation occurs at the parting line between the mold halves. For example, when the force of the injected molten material acting on the surfaces of the core and cavity in opposing directions exceeds a counteracting clamping force exerted by the molding machine, separation can and will occur along the mold parting line. The effect of such separation is the leakage of the pressurized molten material and a resulting undesirable thin film of additional material, known as flash, around the perimeter of the molded article.
There are times however during molding when mold parting line separation occurs intentionally, for example when special processing techniques such as injection-compression are used to reduce molded-in stress in the parts or for example when a pure compression technique is used.
It is known in the prior art to employ a perimeter ring at the parting line to prevent leakage. U.S. Pat. No. 5,720,994 to Asai is illustrative of such an arrangement. The perimeter ring is attached to either the mold core face or the mold cavity face. The ring floats on either a guiding diameter or on pins, and is usually spring loaded away from the mold half to which it is attached. It is restricted to a certain travel or protrusion above the parting line by bolts, tapers or other devices. The ring maintains firm contact with the mating parting line face during mold filling.
To ensure sealing, this prior art arrangement relies on precise alignment of three pieces: the movable mold half, the stationary mold half, and the perimeter ring that slides over their surfaces. Over several million injection cycles, the ring is subject to wear and to seizure or sticking of the ring against the locating surface. If seizure occurs, the ring does not reside in the proper position, and mold flash can result. Misalignment, or non-concentricity, may also occur due to differential thermal expansion between the ring and the core and cavity surfaces.